LeetCode | 34. Find First and Last Position of Element in Sorted Array

34. Find First and Last Position of Element in Sorted Array

Description

描述：https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/description/
题意：在有序数组中，查找目标数的第一次和最后一次出现的下标，没有则返回-1。

Solution: (Java)

class Solution {
    public int[] searchRange(int[] nums, int target) {
        int[] indexs = new int[2];
        int left = 0, right = nums.length-1, mid, first = -1, last = -1;
        indexs[0] = -1;
        indexs[1] = -1;
        while (left <= right) {
            mid = (left + right) / 2;
            if (nums[mid] == target) {
                first = mid;
                last = mid;
                while (first >= 0) {
                    if (nums[first] == target) {
                        indexs[0] = first;
                        first--;
                    } else
                        break;
                }
                while (last <= nums.length-1) {
                    if (nums[last] == target) {
                        indexs[1] = last;
                        last++;
                    } else
                        break;
                }
                break;
            } else if (nums[mid] > target) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return indexs;
    }
}

思路

• 二分法查找，复杂度要求O(log n)；
• 稍微变动二分查找即可，在找到目标之后，往目标的左右两边扩展查找即可；
• 改进：在找到目标后往两边扩展时，还可以继续用二分法。